Unfamiliar trait bound syntax

While looking at the source code for Option::ok_or() I saw a trait bound that looks like E: ~const Drop.

My question is, what does the ~const part mean? I'm assuming it's nightly-only since the rustc_const_unstable attribute on top of the method says as much.

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Wow, that’s a weird syntax, I didn’t even know that exists. (I did know of ~const Trait, but not of this weird abuse of “~const Drop” notation). The compiler apparently isn’t fully aware of the abuse of notation here either; it actually thinks that T: ~const Drop implies T: Drop, and unsoundness entails


And here’s a second issue I found looking into const Drop right now


Oh, that truly looks awful.

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